Different Exporter::shutdown() interface across signals

[lalitb]

Metrics:

opentelemetry-rust/opentelemetry-sdk/src/metrics/exporter.rs

Line 30 in a5e2061

fn shutdown(&self) -> MetricResult<()>;

Trace:

opentelemetry-rust/opentelemetry-sdk/src/export/trace.rs

Line 46 in a5e2061

fn shutdown(&mut self) {}

Logs:

opentelemetry-rust/opentelemetry-sdk/src/export/logs/mod.rs

Line 87 in a5e2061

fn shutdown(&mut self) {}

Logs and Trace are similar, Metrics return the status. Specs doesn't mandates returning status.

This should not be a big deal, just opening the issue to track.